1. Technical Field
The present invention relates generally to an improved structure of electronic parts with solder bumps and a method of soldering electronic parts with solder bumps to a substrate.
2. Background Art
In recent years, electronic parts such as IC chips with solder bumps are in widespread use in electronic equipment for high density arrangement of substrates.
Hereinbelow, a conventional sequence of processes of soldering electronic parts to a substrate will be discussed with reference to FIGS. 8(a) to 8(d).
First, a chip 3 is placed on a board 2. Electrodes of the chip 3 are then bonded to electrodes printed on the board 2 through lead wires 4, respectively. The chip 3 and the wires 4 are encapsulated by an encapsulating member 5. Disposed on a lower surface of the board 2 are solder bumps 6 each bonded to one of the wires 4. The board 2 is placed on a substrate 7 with the solder bumps 6 coinciding with electrodes 8 mounted on a substrate 7, respectively. The encapsulating member 5 is usually molded by transfer molding techniques. In the transfer molding, resin materials of both the board 2 and the encapsulating member 5 are heated above 150.degree. C. and then cooled down to a room temperature at which the resin materials are solidified to form the encapsulating member 5. An epoxy resin and a glass epoxy resin are usually used as materials of the encapsulating member 5 and the board 2, respectively. Thus, the encapsulating member 5 is greater in thermal expansion coefficient than the board 2, so that cooling the encapsulating member 5 and the board 2 will cause the encapsulating member 5 to shrink more greatly than the board 2, thereby leading to the formation of set of an electronic part 1. This causes peripheral ones of the bumps 6 to be separate from the electrodes 8. It is experimentally found by the inventors of this application that the set of the electronic part 1 is cured temporarily by heating the electronic part 1. The test results show that the set of the electronic part 1 begins to be straightened quickly above 183.degree. C. and then cured completely at about 185.degree. C. which will be hereinafter referred to as a thermal deformation temperature. Although the thermal deformation temperature is changed depending upon size, thickness, or material, a typical one will be referred to in the following discussion.
After the electronic part 1 is mounted on the substrate 7 as shown in FIG. 8(a), they are put into a reflow heating furnace and then heated slowly. FIG. 2 shows a temperature profile of the reflow heating furnace. A melting point of the solder bumps 6 is 183.degree. C.
Next, the substrate 7 is heated quickly from the room temperature up to about 150.degree. C. and then heated slowly up to about 170.degree. over a preheating zone shown in FIG. 2. The substrate 7 is heated quickly again over a reflow zone until a melting point of the solder bumps 6 is reached at 183.degree. C. Subsequently, the substrate 7 is further heated up to a point b (thermal deformation temperature) so that the set of the electronic part 1 is, as shown in FIG. 8(b), cured so that it becomes flat.
The substrate 7 is further heated up to a maximum temperature of 230.degree. C. at a point c. FIG. 8(c) shows the electronic part 1 at 230.degree. C. The condition of the electronic part 1 in FIG. 8(c) is substantially identical with that in FIG. 8(b). After the maximum temperature is reached, the substrate 7 is cooled quickly. When the temperature in the heating furnace is decreased below 185.degree. C. at a point d, it will cause the board 2 to be curved again. At 183.degree. C. (point e), the solder bumps 6 are solidified, as shown in FIG. 8(d).
As discussed above, the decrease in temperature of the heating furnace below 185.degree. C. (i.e., the thermal deformation temperature) in the cooling process causes the board 2 to be curved again, as shown in FIG. 8(d), due to the difference in thermal expansion coefficient between the encapsulating member 5 and the board 2. Since the solder bumps 8 are not yet solidified at 185.degree. C., the electronic part 1 is attracted downward by the surface tension of the molten solder bumps 6 and the weight of the electronic part 1, so that the central solder bumps 6 are greatly pressed against the substrate 7 and then expanded laterally, thereby leading to the formation of bridges 9 which cause short-circuits to occur.